A shared "Mouse Core" is proposed as Core Unit C for the program project. The goal of this core is to enable the investigator to perform in vivo studies in a timely and economical manner. The responsibilities of this core are two-fold. First, this core will maintain breeding colonies for B6.SJL-PtrcaPep3b/BoyJ (Boy.!) mice and NOD.CB17-Pr/[unreadable]fciC"'/J-nonobese/severe combined immunodeficient-(NOD/SCID) mice. Separate breeding and experimental rooms for these colonies are conveniently located in the adjacent Medical Sciences buildir g in the Laboratory of Animal Resource Center. The BoyJ mice will be used in a variety of transplantation experiments in which the relative repopulating ability of two genetically distinct sources of stern culls will be studied. In addition these mice will be used to study submyeloablative regimens and strategies to select for low numbers of transduced stem cells in vivo. The NOD/SCID mice will be utilized in investigations focused on human stem and progenitor cell biology in this program. Second, for NOD/SCID experiments, the core leader will manage and coordinate with core personnel all aspects of the transplantation. In addition, core personnel will perform transplants into NOD/SCID mice, harvest and provide tissues from NOD/SCID experiments to investigators of this program project for analysis. This will also be coordinated with the "Cell Purification and Analysis Laboratory (Core A)." The core leader will also provide assistance with experimental design and interpretation of data generated in NOD/SCID xeriograft transplantation experiments. The shared Core A will provide participants in the program project with costeffective and high quality services for their research. These studies will provide new information regarding hematopoietic stem-cell function in vivo that is applicable to the future development of gene therapy trials.